Method of controlling the opening/closing of a mold in an injection molding machine

Abstract

 In a method of controlling the opening/closing of a mold in an injection molding machine to provide increased accuracy, a changeover position indicative of a speed of a movable plate (1) of the mold or the changeover position of a pressure in a cylinder (6) of the mold opening and closing mechanism is preset in a device (11) in accordance with a corresponding position of the movable plate (1). The value thus set is arithmetically converted in a device (10) to a corresponding displacement distance of a piston rod (6a) of the mold-clamping cylinder (6). The converted value is stored in a device (11a) and is compared in a comparator (9) with the actual displacement of the rod as detected by a device (7) and when the two values coincide, the hydraulic pressure controlling the mold is changed appropriately.

Description

[0001] The present invention relates to a method of controlling a mold opening/closing operation in an injection molding machine having a toggle-type mold-clamping mechanism.

[0002] Conventionally, for controlling the speed of a movable plate of the mold or the pressure applied to the movable plate when a mold of an injection molding machine is opened or closed, a method has been employed in which the speed or the pressure is changed depending on the displacement position of the movable plate. A limit switch, potentiometer, or linear encoder has been used to detect the displacement position. Especially when the mold opening and closing operation is centrally and remotely controlled, there has been employed a controlling method wherein a displacement position detecting device such as a potentiometer or a linear encoder is used to detect directly the displacement position of the movable plate, and a signal is thereby generated to change the speed or pressure when the detected value agrees with a preset value.

[0003] This method is frequently employed not only in direct-pressing type mold-clamping systems, but also in toggle-type mold-clamping systems. Although enhancement of the mold-clamping force just before a mold-clamping operation is completed is brought about by a toggle-type mold-clamping mechanism. We have found that it is possible to increase the accuracy in detecting foreign matter sandwiched between the mold parts which are clamped together, if the control is effected by detecting the displacement distance of a rod of a mold-clamping hydraulic cylinder which operates the toggle mechanism by detecting the displacement position of the movable plate. That is because the displacement distance S (stroke) of the rod is much greater than the displacement distance S' (stroke) of the movable plate and thus the accuracy is correspondingly higher if control is performed in accordance with the displacement distance of the rod. The ratio between the displacement distances (S/S') is typically more than about ten, and may be more than about 20 just before the mold parts come into contact with each other.

[0004] In the toggle-type mechansim, there is a correlation between the displacement distance S of the rod and the displacement distance S' of the movable plate. However, as indicated by the graph of figure 2 of the accompanying drawings, the relationship between the displacement distances S and S' is non-linear, and therefore the above-mentioned method in which the displacement distance of the movable plate is directly detected to control the speed or the pressure in the mold opening and closing operation has generally been preferred, similar to that used in the direct pressing type mold-clamping system. In a mold opening/closing control method which employs the advantages of a toggle-type mold-clamping system, there has been additionally provided a detecting device such as a limit switch, a contactless switch, or the like, for detecting the displacement distance S of the rod of the mold-clamping cylinder in order to increase the accuracy in detecting foreign matter. However, such a detecting device must be provided not as a central and remotely controlled unit, but as a unit inside a mold-clamping device part of the injection molding machine.

[0005] Therefore, because in conventional injection molding machines having a toggle-type mold-clamping mechanism the displacement distance of the movable plate is directly detected and controlled in order to control the speed of or pressure applied to the movable plate in mold opening/closing operations, and further because it is necessary to provide another device for detecting the displacement distance of the rod of the mold-clamping cylinder inside the machine, the overall structure of the apparatus is complicated. Furthermore, in toggle-type injection molding machines, it has been proposed to provide cam means and limit switches for detecting an operational movement of a cross link to control the mold opening/closing operation. However, according to this proposal, since the mold movement and the cross link movement are not kept in a linear relationship, it is difficult to effect adjustment and setting of the control mechanism.

[0006] In order to solve the problems of the prior art described above the present invention utilizes the fact that there is a correlation between the displacement distance S of the rod of the mold-clamping cylinder and the displacement distance S' of the movable plate.

[0007] Thus, according to the present invention, a method of controlling the opening and closing of a mold, which includes a movable plate, in an injection molding machine having a toggle-type mold-clamping mechanism which is operated by hydraulic pressure is characterized by the steps of: providing a changeover position setting device for setting one of a speed of the movable plate and a changeover position of a hydraulic pressure supply to the mechanism to a valve in accordance with a corresponding position of the movable plate, arithmetically converting a value set in the setting device into a corresponding displacement distance of a rod of a mold-clamping cylinder of the mechanism; storing the converted value; detecting a displacement distance of the rod; comparing the stored converted value with the detected displacement distance; and changing the hydraulic pressure when the detected displacement distance of the rod and the stored value coincide with each other.

[0008] More specifically and preferably, when the value of the detected displacement distance of the rod of the mold-clamping cylinder agrees with the value in the converted changeover position storage device, an electromagnetic flow adjustment valve or an electromagnetic relief valve in a hydraulic pressure circuit is controlled by a sequencer.

[0009] Thus it will be seen that the displacement distance S' of the movable plate is converted into the displacement distance S of the rod of the mold-clamping cylinder and control is performed by comparing the result of the conversion with the detected value of the actual displacement distance of the rod.

[0010] An example of a method in accordance with the invention will now be described with reference to the accompanying drawings in which:-

Figure 1 is a block diagram showing a preferred embodiment of a control system for carrying out the method in accordance with the present invention; and,

Figure 2 is a graph showing the relationship between the displacement distance of a rod of a mold-clamping cylinder and the displacement distance of a movable plate forming parts of a mold opening and clamp mechanism.

Figure 1 is a block diagram illustrating a preferred embodiment of a control system for carrying out the method according to the present invention.

[0011] A rod displacement distance detecting device 7 for detecting the displacement distance S of a rod 6a of a mold-clamping cylinder 6 is provided between a mold-clamping housing 2 and a cross link 5 connected to the rod 6a. Reference numeral 1 designates a movable plate, 3 a toggle link, and 4 a tie bar for guiding the movable plate 1.

[0012] The control system will be described. Reference numeral 11 designates a changeover position setting device for controlling the speed of or pressure applied to the movable plate 1 in a mold opening/closing operation, the changeover position being determined depending on the displacement position of the movable plate. An arithmetic conversion arithmetic device 10 converts the setting value of the changeover position setting device 11 into a displacement distance S of the rod 6a according to a mathematical expression corresponding to the relationship between the displacement distance S of the rod 6a and the displacement distance S' of the movable plate 1 (which have been stored in advance), and supplies the converted value to a converted changeover position storage device lla. A comparator 9 is supplied with the detected value from the device 7 for detecting the displacement distance of the rod 6a and a converted value of the setting value from the converted changeover position storage device lla. When the values of the two agree, the comparator 9 provides a changeover signal to a sequencer 12. In response to this signal, the sequencer 12 actuates an electromagnetic flow adjusting valve or an electromagnetic relief valve 14 in a hydraulic circuit to operate, through a driver amplifier 13, a valve to control the above-mentioned speed or pressure.

[0013] Since the present invention employs an arrangement as described above, it is not necessary to detect directly the displacement position of the movable plate, and the displacement position of the movable plate can be detected as an amplified value of the displacement position of the mold-clamping cylinder. Hence, the accuracy of changeover control is increased, and it is possible to make the setting of the changeover position on the basis of the displacement position of the movable plate. Further, the accuracy in detecting foreign matter in the mold is improved by using the toggle magnifying ratio, which is a feature of a toggle-type mold clamping mechanism, and all of the control operations described above may be performed centrally and remotely.

Claims

1. A method of controlling the opening and closing of a mold, which includes a movable plate (1), in an injection molding machine having a toggle-type mold-clamping mechanism which is operated by hydraulic pressure, characterised by the steps of: providing a changeover position setting device (11) for setting one of a speed of the movable plate (1) and a changeover position of a hydraulic pressure supply to the mechanism to a valve in accordance with a corresponding position of the movable plate (1), arithmetically converting a value set in the setting device (11) into a corresponding displacement distance of a rod (6a) of a mold-clamping cylinder (6) of the mechanism; storing the converted value; detecting a displacement distance of the rod (6a); comparing the stored converted value with the detected displacement distance; and changing the hydraulic pressure when the detected displacement distance of the rod and the stored value coincide with each other.

2. A method according to claim 1, wherein the step of changing the hydraulic pressure is effected with a sequencer (12) driving a flow adjusting value.

3. A method according to claim 1, wherein the step of adjusting the hydraulic pressure is effected with a sequencer (12) driving an electromagnetic relief valve (14).

4. A method according to claim 1, substantially as described with reference to the accompanying drawings.

Drawing